Semiconductor structure having both enhancement mode group III-N high electron mobility transistors and depletion mode group III-N high electron mobility transistors

ABSTRACT

An Enhancement-Mode HEMT having a gate electrode with a doped, Group III-N material disposed between an electrically conductive gate electrode contact and a gate region of the Enhancement-Mode HEMT, such doped, Group III-N layer increasing resistivity of the Group III-N material to deplete the 2DEG under the gate at zero bias.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Divisional Application of U.S. application Ser.No. 16/379,077, filed Apr. 9, 2019, which application is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

This disclosure relates generally to Group III-Nitride (Group III-N)enhancement mode (E-Mode) High Electron Mobility Transistors (HEMTs) andmore particularly to semiconductor structures having both Group III-NE-Mode HEMTs and Group III-N depletion mode (D-Mode) field effecttransistors (FETs) on a common crystal substrate.

BACKGROUND OF THE INVENTION

As is known in the art, Group III-N high electron mobility transistors(HEMTs) have high breakdown voltages, large electron saturationvelocities, high intrinsic polarization induced two-dimensional electrongas (2DEG) channels, and large conduction band offsets. Group III-Nmaterials in the wurtzite crystal structure exhibit spontaneous andpiezoelectric polarization due in part to structural deviations from theideal tetrahedral coordination along the (0001) axis (c-axis) anddifferences in the electronegativity between the bonded group III andnitrogen atoms. The Group III-N include Indium Nitride (InN), GalliumNitride (GaN), Aluminum Nitride (AlN), Boron Nitride (BN) and all oftheir associated alloys including In_(x)(Al_(y)Ga_(1-y))_(1-x)N (where0≤x≤1 and 0≤y≤1) and B_(z)(In_(x)(Al_(y)Ga_(1-y))_(1-x))_(1-z)N (where0≤x≤1 and 0≤y≤1 and 0≤z≤1); where x+y+z=1. More particularly, as is alsoknow, transistors using the c-axis metal polar orientation of the GroupIII-N materials are typically based on AlGaN/GaN heterostructure betweena lower GaN layer and upper AlGaN layer where at the GaN layer side ofthe AlGaN/GaN heterojunction, or interface, a 2DEG channel forms tocompensate the net polarization charge created by the polarizationdiscontinuity that exists at that heterojunction. In the AlGaN/GaNheterojunction, the AlGaN layer is generally referred to as the topsidebarrier layer separating the heterojunction from the gate electrodewhile the GaN layer serves as the 2DEG channel layer wherein mobilecharge resides. Group III-N HEMTs are typically operated in depletionmode where a negative bias voltage is applied to the gate electroderelative to the source electrode to deplete carriers in the 2DEG channelbelow the gate electrode and turn off conduction between the source anddrain electrodes; the gate electrode being disposed between an ohmicsource electrode and an ohmic drain electrode.

An advantage of a Group III-N depletion mode device is that it can bereadily fabricated using a three terminal device structure whereby thegate electrode is formed between an ohmic source electrode and an ohmicdrain electrode. The channel below the gate region of a Group III-Ndepletion mode device is conductive when the source, drain, and gateelectrodes are all grounded or equivalently held at the same biascondition. The voltage biases on the gate and drain electrode voltagesare quoted relative to the source electrode. Therefore, when a threeterminal Group III-N depletion mode device is operated under a zero biasvoltage condition, an equivalent bias is applied to both the gateelectrode and the source electrode such that no potential exists betweenthe two electrodes. When the drain electrode is biased negatively withrespect to the source electrode and the gate electrode is operated underzero gate bias voltage conditions in a Group III-N depletion modedevice, current will flow between the source and drain electrodes due tothe presence of the 2DEG channel in the structure. As the voltage at thegate electrode becomes more negative with respect to the voltage at thesource electrode, the carriers under the gate electrode will start todeplete and the total current that can be transported between the sourceand drain electrodes will begin to decrease. The minimum gate-to-sourcevoltage (V_(GS)) that is needed to create a conducting path between thesource and drain electrodes is called the threshold voltage.

As is also known in the art, it is sometimes desirable to form bothD-Mode and E-Mode HEMTs on the same crystal body, as in an integratedcircuit chip. In one application, for example, in the event of aspecific type of failure, they need a ‘Fail Safe Switch’ to inherentlyrespond in a way that will cause no or minimal harm to other equipment.Further, it is important to fabricate E-mode GaN-based transistorswithout disturbing and sacrificing the performance of the existingD-mode GaN-based transistors. An E-mode transistor requires a thresholdvoltage equal to, or greater than zero on the gate electrode relative tothe source electrode for current to flow between the source and thedrain electrode and under the gate electrode. More particularly, thereis a need to fabricate stable epitaxial gate structures for Group III-Nenhancement mode fail safe switches on the same wafer as highperformance RF depletion mode devices, with threshold voltages on theenhancement mode gate electrode relative to the voltage on the sourceelectrode greater than +1 Volt. E-mode devices typically need athreshold voltage of at least +1 V to protect the circuit from noise onthe gate signal and the threshold voltage needs to be stable over theoperational lifetime.

E-mode AlGaN/GaN HEMTs with positive and stable threshold voltages havebeen reported using p-type GaN gate electrodes; see a paper by Meneghiniet al., Technology and reliability of normally-off GaN HEMTs with p-typegate, Energies 10, 153, 2017. See also Materials Science inSemiconductor Processing 78 (2018) 96-106. Review of technology fornormally-off HEMTs with p-GaN gate by Giuseppe Greco et al., describingthe use of a Mg doped GaN gate electrode (a p-type doped GaN electrode).See also U.S. Pat. No. 7,728,356 Issued Jun. 1, 2010 Suh et al.,P-GaN/AlGaN/AlN/GaN enhancement-mode field effect transistor. Thesedevices utilize a p-type GaN gate electrodes with a magnesium (Mg) dopedGaN layer between the AlGaN barrier layer and a metal electrode. The Mgdoping provides p-type conductivity in the Mg doped GaN layer thatraises the conduction band at the AlGaN/GaN interface and depletescarriers from the 2DEG channel under the zero bias conditions.

However, the use of Mg to produce the p-type GaN for the gate electrodecreates processing issues in many fabrication facilities because Mgcontaminates many types of processing equipment that can otherwise beused for other processing steps.

As is also known in the art, beryllium doped GaN material displaysinsulating behavior, see K. Lee et al., Compensation in Be-doped GalliumNitride Grown Using Molecular Beam Epitaxy, Material Research SocietySymposium, Proc. Vol. 892 (2006). The insulating characteristics ofberyllium doped GaN layers have been used to mitigate the effects ofconductive buffer layers in GaN HEMTs, see D. F. Storm et al., Reductionof buffer layer conduction near plasma-assisted molecular-beam epitaxygrown GaN/AIN interfaces by beryllium doping, Appl. Phys. Lett., 81,3819, 2002. Theoretical calculations of the actual ionization energy ofsubstitutional beryllium in GaN over the years have estimated theionization energy to be anywhere from as low as 60 meV see Bernardini etal., Theoretical evidence for efficient p-type doping of GaN usingberyllium, arXiv:cond-mat/9610108v2, 1997, to as high as 550 meV see J.L. Lyons et al., Impact of Group-II Acceptors on the Electrical andOptical Properties of GaN, Jpn. J. Appl. Phys. 52, 08JJ04, 2013.Beryllium interstitials in GaN also have been calculated to have a lowformation energy and to be a double donor suggesting berylliuminterstitials are likely to incorporate during growth and lead to thecompensation of substitutional beryllium acceptors in GaN see C. G. Vande Walle et al., First-principles studies of beryllium doping of GaN,Phys. Rev. B, 63, 245205, 2001. The possibility that beryllium mayoccupy interstitial as well as substitutional sites may be anotherpossible reason why beryllium doping does not produce p-typeconductivity.

SUMMARY OF THE INVENTION

In accordance with the present disclosure, an Enhancement-Mode HEMT isprovided having a gate electrode comprising: a layer, disposed betweenan electrically conductive gate electrode contact and a gate region ofthe Enhancement-Mode HEMT, such layer comprising: a Group III-Nmaterial, the Group III-N material having a predetermined resistivityand a dopant disposed in the Group III-N material, such dopant:providing the layer with a resistivity greater than the predeterminedresistivity of the Group III-N material; and depleting carriers from a2DEG under the gate at zero gate bias.

In one embodiment, an Enhancement-Mode HEMT is provided having a gateelectrode comprising: a layer, disposed between an electricallyconductive gate electrode contact and a gate region of theEnhancement-Mode HEMT, such layer comprising: a Group III-N material,the Group III-N material having a predetermined resistivity and a dopantdisposed in the Group III-N material, such dopant: providing the layerwith a resistivity greater than the predetermined resistivity of theGroup III-N material, and depleting carriers from a 2DEG under the gatewhen an applied gate voltage is less than a threshold voltage, and thethreshold voltage is equal to, or greater than zero.

In one embodiment, the dopant is beryllium.

In one embodiment, the dopant is Molecular Beam Epitaxy beryllium.

In one embodiment the doped Group III-N material comprises GaN or AlGaN.

In one embodiment, an Enhancement-Mode HEMT structure is providedhaving: a crystal structure having a pair of stacked Group III-Nsemiconductor layers, the pair of stacked Group III-N semiconductorlayers forming a heterojunction with a 2DEG channel being formed in alower one of the pair of stacked Group III-N layers; a source electrodefor supplying current to the 2DEG; a drain electrode for extractingcurrent supplied from the 2DEG; and a gate electrode, disposed betweenthe source electrode and the drain electrode and over a gate region ofthe upper one of the pair of stacked layers for controlling the suppliedcurrent passing to the drain electrode; wherein such gate electrode isdisposed over the gate region. The gate electrode, comprising: anelectrically conductive gate electrode contact; a doped, Group III-Nmaterial, disposed between the electrically conductive gate electrodecontact and the gate region, such doped, Group III-N material increasingresistivity of the Group III-N material and provides the HEMT with athreshold voltage equal to, or greater than zero.

In one embodiment, the doped, Group III-N material forces the Fermilevel in the doped Group III-N material to reside close enough to thevalance band edge to raise the conduction band at the interface betweenthe pair of stacked Group III-N semiconductor layers to deplete carriersfrom the 2DEG channel under the gate region at zero gate bias.

In one embodiment, the doped, Group III-N material is grown by MolecularBeam Epitaxy (MBE).

In one embodiment, the doped, Group III-N material is grown under GroupIII rich surface conditions by Molecular Beam Epitaxy (MBE).

In one embodiment, the gate electrode comprises a single doped, GroupIII-N material.

In one embodiment, the doped, Molecular Beam Epitaxy Group III-Nmaterial comprises Beryllium.

In one embodiment, an Enhancement-Mode HEMT structure is provided havinga gate electrode with a doped, Group III-N material disposed between anelectrically conductive gate electrode contact and a gate region of theEnhancement-Mode HEMT structure, such doped, Group III-N layerincreasing resistivity of the Group III-N material and depleting the2DEG under the gate region at zero bias.

In one embodiment, a method is provided for forming an Enhancement modeHEMT, structure having an AlGaN/GaN structure to produce a 2DEG in theGaN portion of the AlGaN/GaN structure, the method comprising: forming agate structure for Enhancement mode HEMT structure comprising: aberyllium doped, molecular beam epitaxy layer formed under gallium-richgrowth conditions to produce resistive material that shifts the bandstructure in an AlGaN/GaN HEMT to produce positive threshold voltagesrequired for E-mode operation.

In one embodiment, the beryllium doped Group III-N layer is grown by MBEwith a predetermined gallium to nitrogen flux ratio selected to maintainmore than a monolayer of liquid gallium on the surface during the MBEgrowth.

In one embodiment, an Enhancement-Mode HEMT is provided having a gateelectrode with a doped, Group III-N material disposed between anelectrically conductive gate electrode contact and a gate region of theEnhancement-Mode HEMT, such doped, Group III-N layer increasingresistivity of the Group III-N material and depleting carriers from a2DEG under the gate when an applied gate voltage is less than athreshold voltage, and the threshold voltage is equal to, or greaterthan zero.

In one embodiment, a structure, comprising: a single crystal substrate;a Depletion mode (D-mode) HEMT and an Enhancement mode (E-mode) HEMTformed on the single crystal substrate; the Enhancement-Mode HEMT havinga gate electrode with a doped, Group III-N material disposed between anelectrically conductive gate electrode contact and a gate region of theEnhancement-Mode HEMT, such doped, Group III-N layer increasingresistivity of the Group III-N material and depleting carriers from a2DEG under the gate when an applied gate voltage is less than athreshold voltage, and the threshold voltage is equal to, or greaterthan zero.

In one embodiment, a method is provided for forming an Enhancement-ModeHEMT, comprising: forming a gate electrode comprising: a layer, disposedbetween an electrically conductive gate electrode contact and a gateregion of the Enhancement-Mode HEMT, such layer comprising: a GroupIII-N material, the Group III-N material having a predeterminedresistivity; and depositing by molecular beam epitaxy, a dopant in theGroup III-N material, such dopant: providing the layer with aresistivity greater than the predetermined resistivity of the GroupIII-N material; and depleting carriers from a 2DEG under the gate atzero gate bias.

In one embodiment, a method is provided for forming an Enhancement-ModeHEMT, comprising: forming a gate electrode comprising: a layer, disposedbetween an electrically conductive gate electrode contact and a gateregion of the Enhancement-Mode HEMT, such layer comprising: a GroupIII-N material, the Group III-N material having a predeterminedresistivity; a depositing by molecular beam epitaxy, a dopant in theGroup III-N material, such dopant: providing the layer with aresistivity greater than the predetermined resistivity of the GroupIII-N material, and depleting carriers from a 2DEG under the gate whenan applied gate voltage is less than a threshold voltage, and thethreshold voltage is equal to, or greater than zero.

The inventors, as a result of their experimentally generated data,recognized, that in spite of the teaching of Meneghini et al., and Grecoet al., that p-type doped GaN (Mg) for the gate electrode (dopant thatthat will reduce the resistivity of the GaN) can be used to depletecarriers in the 2DEG channel under zero bias conditions; Applicant usesa more process friendly dopant, e.g., beryllium, which, while having theopposite effect on the resistivity of GaN (i.e., the beryllium, whichincreased the resistivity of the GaN as distinguished from Mg whichreduced the resistivity of the GaN) was able to deplete carriers in the2DEG channel under zero bias conditions.

The inventors first questioned whether the GaN for the gate electrodewas required to have mobile p-type carriers, or whether it wassufficient to simply adjust the Fermi level in the GaN for the gateelectrode. The inventors recognized that beryllium GaN might pin theFermi level at an acceptor level, based on the various calculations ofionization energy (Bernardini et al., Lyons et al., and Van de Walle etal., referenced above), but likely at a higher energy level than Mgbased on the lack of p-type conductivity in beryllium doped GaN. Suchrecognition led to the first few experiments where the inventors wereable to show that beryllium doped GaN for the gate electrode couldactually deplete a 2DEG in an AlGaN/GaN HEMT when grown by MBE underGa-rich surface conditions. Prior to the experiments, the inventors didnot know if beryllium doped GaN would be able to deplete an AlGaN/GaN2DEG as there was no ability to know based on the prior art that theinventors were, and are today, aware of as to exactly where and howefficiently beryllium doped GaN would pin the bands for E-modeoperation; in fact, even based on the success of the experiments theinventors still do not know exactly where the bands are pinned. Onceberyllium doped GaN was shown to deplete an AlGaN/GaN 2DEG, theinventors still did not know a priori what level of threshold voltagecould be realized until they processed and measured the first transistorstructures. In this way, the inventors, as a result of theirexperimentally generated data, recognized that one does do not needp-type conductivity in GaN to alter the carrier concentration in a 2DEGas taught in the prior art described above, but rather one needs to beable to sufficiently pin the Fermi level as can be done with a resistiveGaN material to alter the carrier concentration in a 2DEG and thereforeuse beryllium as a dopant for the GaN; the use of beryllium in GaN beinga more process friendly dopant to use than Mg. More particularly, in oneembodiment, the beryllium having a doping concentration of 5×10¹⁸/cm³was experimentally found by the inventors to reduce the resistivity ofthe GaN from 100 Ohm-cm for undoped GaN to 2.2×10³ Ohm-cm for theberyllium doped GaN and depletes carriers from a 2DEG under the gate atzero gate bias.

To put it still another way, the inventors have disregarded theconflicting teachings of the prior art discussed above and of which theinventors were, or are today, aware of as to where the ionization energyof beryllium doped GaN lies with respect to the valance band or whetherit is highly compensated or not as in the above described publicationsand experimentally determined and produced E-mode HEMTs with berylliumdoped GaN gates and determined that the beryllium doped GaN in theproduced E-mode HEMTs did not have p-type conductivity, but rather wasinsulating. The inventors then realized that in a p-GaN gate E-mode GaNHEMT reported in for example Greco et al., the p-type conductivity ofthe doped GaN was not responsible for depleting the carriers in the 2DEGunder zero bias, rather it was where the doping pinned the Fermi levelwith respect to the valance band of the GaN that was important.Therefore, despite the fact that beryllium doped GaN does not havep-type conductivity, and the actual Fermi pinning level of the berylliumdopant is not known to the inventors even today the inventors have, as aresult of their experimentally generated data, recognized that aresistive beryllium doped GaN layer under the gate region of a GaN HEMTis an alternative material for fabricating E-mode GaN devices.

The inventors thereby also recognized that by providing a layer of GaNdoped beryllium by molecular beam epitaxy under gallium-rich surfaceconditions produces resistive material that shifts the band structure inan AlGaN/GaN HEMT to produce positive threshold voltages required forE-mode operation. The inventors further recognized that similar E-modeoperation of GaN HEMTs realized with p-type GaN under the transistorgate can still be realized if resistive GaN is used in place of thep-type GaN as long as the Fermi level is pinned close enough to thevalance band edge of the resistive GaN to deplete all the carriers inthe 2DEG at zero bias conditions. Metal-rich surface conditions occurduring MBE growth of Group III-N when a predetermined Group III tonitrogen flux ratio is used to maintain more than a monolayer excess ofthe Group III elements on the surface during the growth. Metal richsurfaces typically reduce oxygen incorporation and promote smoothersurfaces than nitrogen rich growth surfaces in MBE growth. Berylliumdoping in GaN from 1×10¹⁸/cm³ to 1×10¹⁹/cm³ under gallium rich surfaceconditions is possible for E-mode applications as higher doping levelsbegin to create additional defects and disorder in the material andeventually lead to structural degradation around 5×10¹⁹/cm³ while dopinglevels below 1×10¹⁸/cm³ become inefficient at shifting the bandstructure to realize E-mode operation. Further, the vapor pressure ofberyllium is low leading to no undesired background doping or chambermemory effects for MBE growth. Still further, the growth of berylliumdoped Group III-N under the gate region by MBE also enables dual E-modeand D-mode devices to be realized on the same wafer.

Thus, the inventors, as a result of their experimentally generated data,recognized that the mobile hole carriers in Mg doped GaN are notrequired to make an E-mode device and they, as a result of theirexperimentally generated data, recognized that beryllium doping bymolecular beam epitaxy under gallium-rich growth conditions producesresistive material that is capable of shifting the band structure in anAlGaN/GaN HEMT to produce positive threshold voltages required forE-mode operation. To put it another way, the inventors recognized thatthe benefits of the p-type GaN used under the transistor gate for E-modestructures can still be realized if resistive GaN is used in place ofthe p-type GaN as long as the Fermi level is pinned close enough to thevalance band edge of the resistive GaN to deplete all the carriers in a2DEG at the AlGaN/GaN interface under zero bias conditions.

Thus, the use of beryllium dopant in place of Mg doped GaN E-mode HEMTsremoved the problem of having to use Mg doping to produce E-mode HEMTswith its potentially damaging effects on many types of processingequipment that can otherwise be used for other processing steps.

The details of one or more embodiments of the disclosure are set forthin the accompanying drawings and the description below. Other features,objects, and advantages of the disclosure will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagrammatical sketch of a structure having botha D-Mode HEMT and an E-Mode HEMT according to the disclosure;

FIG. 2A is a structure without beryllium doped GaN and a plot ofcapacitance versus voltage of the structure showing the relative chargein a 2DEG of the structure useful in understanding the E-mode HEMT ofFIG. 1 ;

FIG. 2B is a structure with a 500 Å beryllium doped GaN layer and a plotof capacitance versus voltage of the structure showing the relativecharge in a 2DEG of the structure useful in understanding the E-modeHEMT of FIG. 1 ;

FIG. 2C is a structure and a set of capacitance-voltage measurements fordifferent Al_(0.25)Ga_(0.75)N layer thickness of the structure showingthe relative charge in the 2DEG at the interface of an AlGaN/GaNepitaxial material structure terminated with a 500 Å beryllium doped GaNlayer grown by MBE useful in understanding the E-mode HEMT of FIG. 1 ;

FIG. 2D is a structure and a set of capacitance-voltage measurementstaken on the surface of an epitaxially grown Group III-N structureterminated with an undoped GaN layer, a 150 Å Al_(0.25)Ga_(0.75)N layer,and a 150 Å beryllium doped GaN layer grown by MBE useful inunderstanding the E-mode HEMT of FIG. 1 ;

FIG. 2E is a schematic of a three terminal AlGaN/GaN HEMT having a GaNchannel layer, an Al_(0.25)Ga_(0.75)N layer, ohmic contact pads forsource and drain electrodes, a Schottky gate metal contact, and a 500 Åberyllium doped GaN layer located directly below the gate metal and indirect contact with the AlGaN layer; and a plot of the source-to-draincurrent versus gate-to-source voltage of the three terminal AlGaN/GaNHEMT for different Al_(0.25)Ga_(0.75)N layer thicknesses useful inunderstanding the E-mode HEMT of FIG. 1 ;

FIG. 2F is a sketch of a plan view of a mercury probe contact geometryused to generate the plots in FIGS. 2A-2E;

FIGS. 3A-3F are simplified diagrammatical sketches of the structure ofFIG. 1 having both a D-Mode HEMT and an E-Mode HEMT of FIG. 1 at variousstages in the manufacture thereof according to the disclosure;

FIG. 4 is a simplified diagrammatical sketch of a structure having botha D-Mode HEMT and an E-Mode HEMT according to an alternative embodimentof the disclosure;

FIGS. 4A-4D are simplified diagrammatical sketches of the structure ofFIG. 4 having both a D-Mode HEMT and an E-Mode HEMT of FIG. 4 at variousstages in the manufacture thereof according to an alternative embodimentof the disclosure;

FIG. 5 is a simplified diagrammatical sketch of a structure having botha D-Mode HEMT and an E-Mode HEMT according to an alternative embodimentof the disclosure;

FIGS. 5A-5H are simplified diagrammatical sketches of the structure ofFIG. 5 having both a D-Mode HEMT and an E-Mode HEMT at various stages inthe manufacture thereof according to an alternative embodiment of thedisclosure;

FIG. 6 is a simplified diagrammatical sketch of a structure having botha D-Mode HEMT and an E-Mode HEMT according to an alternative embodimentof the disclosure;

FIGS. 6A-6C are simplified diagrammatical sketches of the structure ofFIG. 6 having both a D-Mode HEMT and an E-Mode HEMT at various stages inthe manufacture thereof according to an alternative embodiment of thedisclosure; and

FIG. 7 is a simplified diagrammatical sketch of a structure having botha D-Mode HEMT and an E-Mode HEMT according to an alternative embodimentof the disclosure.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring now to FIG. 1 , a semiconductor structure 10 is shown having adepletion mode (D-mode) field effect transistor 12, here a D-mode HEMT,disposed in one portion of the semiconductor structure 10, and anenhancement mode (E-mode) field effect transistor 16, here a E-modeHEMT, disposed on another portion of the semiconductor structure 10structure laterally positioned adjacent to the depletion mode fieldeffect transistor 12, as shown. The depletion mode (D-mode) field effecttransistor 12 and the enhancement mode (E-mode) field effect transistor16 are isolated by an isolation region 25, here either an etched regionseparating the two sections into as mesas or by ion implanted particles.

The D-mode HEMT 12 includes a source electrode 26, a drain electrode 28and a gate electrode 34 disposed between the source electrode 26 and thedrain electrode 28, as shown. The E-mode HEMT 16 includes a sourceelectrode 36, a drain electrode 38, and a gate electrode 40 disposedbetween the source electrode 36 and the drain electrode 38, as shown.

More particularly, the semiconductor structure 10 includes a singlecrystal substrate 18, here for example Silicon Carbide (SiC), and anepitaxially grown Group III-N structure, here a pair of stacked,epitaxially grown Group III-N structure semiconductor layers 20, 22, 24;layer 20 being one or more epitaxial grown Group III-N materials formingnucleation and buffer regions of a HEMT structure, layer 22 beingepitaxially grown undoped Group III-N channel materials with lowerresistivity than the layer 20 materials, here for example GaN, and layer24 being epitaxially grown Group III-N barrier material, here forexample AlGaN, forming a heterojunction with 2DEG channel (indicated bydotted line 23) in the GaN layer 22. It is noted that the layers 18, 20,22 and 24 extend laterally under both the D-mode HEMT 12 and the E-ModeHEMT 16; however, as will be described in more detail below, the portionof the 2DEG under the gate electrode 40 of the E-mode HEMT 16 will bedepleted of carriers under a zero bias condition on the E-mode HEMT gateelectrode 40. The gate electrode 40 includes a beryllium doped GaN layer42 a in direct contact with a gate region 42 of the AlGaN layer 24 andan electrically conductive gate contact 42 b in direct contact with theberyllium doped GaN layer 42 a. Here, an electrically conductive gatecontact 42 b is formed as a sequence of metal depositions to form aSchottky contact to the Beryllium doped GaN layer 42 a

More particularly, referring to FIG. 2A, a structure 100 is shown in theupper portion of FIG. 2A having a GaN channel layer 102, and an AlGaNlayer 104, here 120 Å thick, forming a heterojunction structure with a2DEG being produced in the GaN layer 102, as shown. The lower portion ofFIG. 2A is a plot of capacitance measurements of the structure 100obtained from a mercury probe capacitance-voltage (Hg CV) measurementtool taken on the surface of the epitaxially grown Group III-N structure100 terminated with the GaN layer 102 being undoped and the AlGaN layer104 being 120 Å thick Al_(0.25)Ga_(0.75)N. Polarization differencesbetween the GaN layer 102 and the AlGaN layer 104 resulted in theaccumulation of electrons near the interface of the two layers 102, 104and the formation of a 2DEG, as indicated. The Hg CV plot shown in thebottom portion of FIG. 2A was obtained by placing two Hg metal contactson the upper surface of the AlGaN layer 104, one of the contacts being asmall circular dot and the second contact being a much larger ring thatsurrounds the majority of the periphery of the small contact, andelectrically insulated from, the smaller contact, as shown in FIG. 2F. Anegative direct current (DC) voltage was applied to the small contactand the larger ring contact was kept at ground. At zero applied DC bias,the measured capacitance (C) at zero volts can be modeled as a parallelplate capacitor with the contact area defined by the Hg dot size and theseparation determined by the location of the 2DEG below the uppersurface of the AlGaN layer 104. As the magnitude of the negative DCvoltage increases, the carriers in the 2DEG begin to deplete until athreshold voltage V_(TH) is reached, the 2DEG is fully depleted, and thecapacitance drops by several orders of magnitude. The total charge inthe 2DEG in the GaN layer 102 was measured qualitatively by calculatingthe area under the curve 110, which to a rough approximation is equal tothe zero-volt capacitance multiplied by threshold voltage V_(TH). If thearea of the Hg dot contact is well known, the actual charge can becalculated from the area under the curve 110, although in practice HallEffect measurements are typically used for quoting sheet densities. Forthe structure 100, a charge density of 6.6×10¹²/cm² was obtained from acontactless Hall Effect measurement with a sheet resistivity of 500Ω/sq.

As an example of how effective the beryllium doped GaN is in depletingthe 2DEG charge, a 500 Å layer of beryllium doped GaN 108 (FIG. 2B) wasgrown on the surface of the AlGaN layer 104. Structure 100 (FIG. 2A) andstructure 100′ (FIG. 2B) were separate growths with all of therespective layers being deposited at one time in an MBE reactor withoutany interruption at the layer interfaces. The beryllium, gallium, andaluminum were all deposited from effusion cells and the nitrogen wassupplied from a commercial RF plasma source. The growth temperature forthe beryllium doped GaN layer 108 (FIG. 1B) was typically between 725°C. and 750° C. Beryllium was simultaneously deposited with the galliumand nitrogen sources at a GaN growth rate of around 1 Å/sec. The dopingdensity of the beryllium was targeted at 6×10¹⁸/cm³ as determined byprior secondary ion mass spectroscopy calibrations of the growth system.The group III to group V flux ratio was kept group III rich so that thelayer was formed in a metal rich growth regime with excess gallium onthe surface. At the end of the growth the excess Ga was thermallydesorbed from the surface.

Attempts to capture a Hg probe CV measurement plot, shown in the lowerportion of FIG. 2B, on the surface of structure 100′ resulted in nomeasureable capacitance values above the background noise of anInductance-Capacitance-Resistance (LCR) meter, as shown by the curve110′, including the zero volt capacitance measurement. The inability tomeasure a CV trace is consistent with the measurement of a samplewithout a 2DEG indicating the 2DEG in structure 100′ is fully depletedof carriers with the addition of the 500 Å beryllium doped GaN layer 108grown by MBE. The Hg probe CV measurement cannot be used to measure apositive threshold voltage as only the area under the Hg dot contact isconductive at a positive DC voltage, but the measurement does identifywhen the threshold voltage of a HEMT structure is greater than zero. Asheet resistivity greater than 40,000 Ω/sq. was measured for thestructure 100′ by a Lehighton sheet resistivity wafer mapping tool,Lehighton Electronics, Inc. Lehighton, Pa. 18235-0328 and contactlessHall Effect measurements could not be obtained for the structure 100′due to the high sheet resistivity.

FIG. 2C shows, in the lower portion of FIG. 2C, a set ofcapacitance-voltage measurements 110″ 110′″ for differentAl_(0.25)Ga_(0.75)N layer 104 thickness (T) showing the relative chargein the 2DEG at the interface of an AlGaN/GaN epitaxial materialstructure 100″ shown in upper portion of FIG. 2C terminated with a 500 Åberyllium doped GaN layer 108″ grown by MBE. When the AlGaN layer 104 isgrown 220 Å thick, there is considerable charge remaining in the 2DEGdespite the addition of the 500 Å beryllium doped GaN layer 108″ as seenby the area remaining under the CV curve 110″ and a measurable sheetresistivity of 1,900 Ω/sq. When the thickness of the AlGaN layer 104 isreduced to 180 Å, the area under the corresponding CV curve 110′″ islikewise reduced and the sheet resistivity increases to 9,100 Ω/sq. Whenthe thickness of the AlGaN layer 104 is reduced to 150 Å or thinner nomeasureable capacitance values above the background noise of the LCRmeter can be detected and sheet resistivity measurements of over 40,000Ω/sq. are measured indicating the 2DEG is fully depleted for these AlGaNlayer 104 thicknesses. FIG. 2C illustrates there is a limit to theamount of charge that can be depleted from the 2DEG using a berylliumdoped GaN layer and that an Al_(0.25)Ga_(0.75)N thickness of less than180 Å is required to produce a positive threshold voltage required foruse in an enhancement mode device.

FIG. 2D, shows, in the lower portion of FIG. 2D, a capacitance-voltagemeasurement taken on the surface of an epitaxially grown Group III-Nstructure 100′″ shown in the upper portion of FIG. 2D terminated with anundoped GaN layer 102, a 150 Å Al_(0.25)Ga_(0.75)N layer 104, and a 150Å beryllium doped GaN layer 108′ grown by MBE. The doping density of theberyllium was targeted at 6×10¹⁸/cm³ as determined by prior secondaryion mass spectroscopy calibrations of the growth system. The group IIIto group V flux ratio was kept group III rich so that the layer wasformed in a metal rich growth regime with excess gallium on the surface.At the end of the growth the excess Ga was thermally desorbed from thesurface. The negative threshold voltage and area under the CV curve110″″ indicates that the 150 Å beryllium doped GaN layer 108′″ is notsufficient to fully deplete the carriers from the 2DEG. Increasing thethickness of the beryllium doped GaN layer 108′″ to 250 Å is sufficientto fully deplete all of the carriers in the 2DEG and no measureablecapacitance values above the background noise of the LCR meter can bedetected using the Hg probe CV tool. The transition from negative topositive threshold voltages occurs when the beryllium doped GaN layer108′″ is somewhere between 150 Å and 250 Å at a beryllium doping levelof 6×10¹⁸/cm³. Additional structures were grown with the beryllium dopedGaN layer 108′″ as thick as 350 Å and 500 Å with no measureablecapacitance values above the background noise of the LCR meter detectedusing the Hg probe CV tool.

The beryllium doped GaN layer grown under metal rich surface conditionsappears to be efficient at pinning the Fermi level near the valance bandas only 6×10¹⁸ Be atoms/cm³ is required in a 250 Å GaN layer to lift theconduction bands of the 2DEG above the Fermi level.

FIG. 2E, shows, in the left portion of FIG. 2E, a schematic of a threeterminal AlGaN/GaN HEMT 100″″ having a 2DEG in the GaN layer 102, anAl_(0.25)Ga_(0.75)N layer 104, ohmic contacts for source and drainelectrodes, a gate electrode 120 having a Schottky gate metal contact122, and a 500 Å beryllium doped GaN layer 108″″ located directly belowthe gate metal contact and in direct contact with the AlGaN layer 104.Three different versions of the HEMT 100″″ were fabricated with eachversion having a different thickness for the Al_(0.25)Ga_(0.75)N layer104. The fabrication started with three blanket epi growths of the GroupIII-N material layers in an MBE system. The growths all included anundoped GaN channel layer 102, an AlGaN barrier layer 104, and aberyllium doped GaN layer 108″″ grown under metal rich surfaceconditions. At the conclusion of each growth the excess Ga was thermallydesorbed from the surface. The GaN growth rate was around 1 Å/sec forall three layers and the doping density of the beryllium doped GaN layer108″″ was targeted at 6×10¹⁸/cm³ for each growth as determined by priorsecondary ion mass spectroscopy calibrations of the growth system. Theonly difference in the three growths was the length of time the AlGaNlayer 104 was grown. The resulting thickness of the AlGaN layer 104 forthe three growths was 180 Å, 150 Å, and 120 Å, see the left side portionof FIG. 2E.

The processing used to form the device 100″″ was identical for all threewafers. The gate electrode structure 120 was lithographically patternedon the wafer and the beryllium doped GaN layer 108″″ was removed fromthe wafers using a plasma etch process except for the regions locateddirectly under where the gate metal 122 was to be deposited. A mesaisolation etch was performed to isolate different devices. Ohmic metalcontacts were then patterned, deposited, and annealed followed by thedeposition of the gate metal 122 on the areas of the beryllium doped GaNlayer 108″″ that were not etched. The right side portion of FIG. 2Eshows a plot of source-drain current versus applied gate voltage fromthree terminal devices measured on each of the three wafers. The deviceswere single finger gate transistors with 250 micron gate widths andmeasured with 10V applied across the source and drain electrodes. Thesource-drain current 124 as a function of gate voltage for the devicefabricated from the growth with the 180 Å AlGaN layer 104 shows that thesource-drain current 124 begins to increase above zero at around −0.2Von the gate demonstrating weak depletion mode operation. A zero gatevoltage position 130 on the plot is indicated by a vertical line 130. Asource-drain current 126 as a function of gate voltage for the devicefabricated from the growth with the 150 Å AlGaN layer 104 shows that thesource-drain current 126 begins to increase above zero at around +0.9Von the gate demonstrating E-mode operation. A source-drain current 128as a function of gate voltage for the device fabricated from the growthwith the 120 Å AlGaN layer 104 shows that the source-drain current 128begins to increase above zero at around +1.6V on the gate demonstratingE-mode operation well in excess of 1.0V with the use of the MBE grownberyllium doped GaN layer 108′.

For the integration of the E-mode HEMT with D-mode devices on the samewafer, it is important that the beryllium doped GaN layers 108 can beregrown on an AlGaN layer 104 after a series of processing steps andstill provide positive threshold voltages. To demonstrate this point, a120 Å Al_(0.25)Ga_(0.75)N/GaN structure was grown by MBE, removed thestructure from vacuum, patterned photoresist on the surface of the AlGaNlayer 104, stripped the photoresist off the AlGaN layer 104, cleaned thesurface of the AlGaN layer 104 for 30 seconds with 1 HF:100H₂O followedby 1 minute with 1 HCl:10H₂O, placed the structure back into the MBEsystem, heated the sample to 760° C., performed a series of Ga cleans,lowered the temperature to between 725° C. and 750° C., and grown a 500Å layer of 6×10¹⁸/cm³ beryllium doped GaN under Ga-rich surfaceconditions on the AlGaN layer 104. The excess Ga on the surface wasremoved by thermal desorption and the structure was cooled and removedfrom vacuum. Hg probe CV measurements of the regrown structure show nomeasureable capacitance values above the background noise of the LCRmeter, indicating a positive threshold voltage for a structure withregrown beryllium doped GaN is achievable.

Referring again to FIG. 1 , and considering first the D-Mode HEMT 12(FIG. 1 ), source and drain electrode 26, 28 thereof are formed in ohmiccontact to the GaN channel layer 22 with source and drain contactregions 30, 32, respectively of the AlGaN layer 24. The D-Mode HEMT 12is here formed using any conventional processing to provide a D-ModeHEMT having any desired electrical characteristic. It is noted that theAlGaN layer 24 is on, and in direct contact with the GaN layer 22 toform the 2DEG channel for a flow of carriers passing laterally throughthe GaN channel layer indicated by the dotted line 23 in the GaN channellayer 22 between the source region 30 of the AlGaN layer 24 the drainregion 32 of the AlGaN layer 24 of the depletion mode field effecttransistor 12, such flow of carriers being controlled by the gate 34disposed on the AlGaN layer 24 of the depletion mode field effecttransistor 12 between the source region 30 and the drain region 32.

The E-Mode HEMT 16 includes: source and drain electrode 36, 38, formedin ohmic contact to the GaN channel layer 22 with source and draincontact regions 44, 46, respectively of the AlGaN layer 24; and a gateelectrode 40 with a Schottky metal contact 42 b in contact with aberyllium doped GaN layer 42 a in direct contact with the AlGaN layer24, the GaN gate contact region 42 having a lower bottom portion 42 aextending into an upper portion of the AlGaN layer 24 and an upperportion 42 a extending above the upper portion of the of the AlGaN layer24, as shown.

The D-Mode HEMT 12 and E-mode HEMT 16 are electrically isolated from oneanother by ion implantation or etched regions surrounding eachstructure. The semiconductor structure 10 also may include alignmentmarkers 53, as shown, to assist with processing.

Referring now to FIGS. 3A-3F, a semiconductor structure having thesingle crystal substrate 18, semiconductor buffer layer 20, and the pairof stacked semiconductor layers 22, 24, is provided, as shown in FIG.1A,

Next, the mask alignment marker 53, here, for example a refractory metalmarker, is formed on the surface of the structure. The mask alignmentmarker 53 has been placed between the regions where the D-mode HEMT 12and the E-Mode HEMT are to be formed, as shown FIG. 3B, but the actuallocation of alignment markers can be anywhere within a reticle or placedas needed across a wafer.

Next, and referring to FIG. 3C, a hard mask 52, more particularly, amask of a non-reactive material, for example, a dielectric such asSiN_(x), Al₂O₃, SiO₂ or stable metal or stacked combination that isnon-reactive to the underlying AlGaN layer 24 at a temperature of asubsequent MBE process at, for example, 700° C. For example, Al or Tiwould react at 700° C. and degrade if not completely eliminate the 2DEG23 under the Al or Ti, whereas SiN_(x) or SiO₂ will not alter the 2DEG23 under the SiN_(x) or SiO₂ by more ±10% at MBE growth temperatures.

Next, and referring to FIG. 3D, the hard mask 52 is lithographypatterned to have a window 54 formed therein region where the gateelectrode region 42 (FIG. 1 ) of the E-mode HEMT gate electrode 40 is tobe formed. The masked surface of the AlGaN layer 24 is etched by a dryplasma etch process to form a recessed gate trench 56 in the upperportion of the AlGaN layer 24 so that only 50 Å to 180 Å of the AlGaNlayer 24 remains, which is typically 250 Å thick prior to the etch.

Next, referring to FIG. 3E, a doped Group III-N material is deposited onand through the openings in the patterned hard mask layer 52. Here, theGroup III-N material is GaN and the dopant is beryllium. Here forexample 250 Å to 500 Å of beryllium doped GaN (Be:GaN) is depositedusing molecular beam epitaxy equipment, thereby deposing apolycrystalline Be:GaN material 42 p on the upper surface of the hardmask 52 and forming the single crystal, epitaxial grown Be:GaN gateelectrode layer 42 a in the patterned window 54, as shown.

Here, in this embodiment, the beryllium had a doping concentration of5×10¹⁸/cm³ in the GaN and was experimentally found by the inventors toreduce the resistivity of the GaN from 100 Ohm-cm for undoped GaN to2.2×10³ Ohm-cm for the beryllium doped GaN. Further, the Be:GaN material42 a depletes carriers from a 2DEG under the gate region 42 at zero gatebias as denoted by the removal of the dashes in the depiction of the2DEG 23 in FIG. 3E.

Next, and referring to FIG. 3F, the hard mask 52 is lifted off alongwith the deposited polycrystalline Be:GaN material 42 p leaving thesingle crystal, epitaxial grown Be:GaN gate region 42 a in the window 54and in the recessed gate trench 56, as shown.

Having formed the epitaxial Be:GaN gate region 42, the D-Mode HEMT 12(FIG. 1 ) and E-mode HEMT 16 are completed using any conventionalprocesses.

Referring now to FIG. 4 a simplified diagrammatical sketch of asemiconductor structure 10′ having both the D-Mode HEMT 12 and an E-ModeHEMT 16′ according to an alternative embodiment is shown. Here, afterforming the structure shown, and described above in connection with FIG.3C a hard mask 52′ is lithography patterned to have a window 54′ formedtherein region where the E-mode HEMT 16′ is to be formed. The surface ofthe exposed AlGaN layer 24 underneath the dry mask window 54′ ispartially etched using a dry plasma etching process so that only 50 Å to180 Å of the AlGaN layer 24 remains in an AlGaN recess region 56′ afteretching (FIG. 4A). The AlGaN layer 24 is typically around 250 Å thickprior to the etching of the layer for the fabrication of the D-Mode HEMT12 and the thickness of the AlGaN layer 24 must be reduced to below 180Å in the region where E-mode HEMT 16′ is to be formed to produce apositive threshold voltage.

Next, referring to FIG. 4B, a doped Group III-N material is deposited onand through the openings in the patterned hard mask layer 52′. Here forexample 250 Å to 500 Å of beryllium doped GaN (Be:GaN) is depositedusing molecular beam epitaxy equipment, thereby deposing apolycrystalline Be:GaN material 42′p on the upper surface of the hardmask 52′ and forming the single crystal, epitaxial grown Be:GaN gateelectrode layer 42′a in the patterned window 54′, as shown. The Be:GaNmaterial 42′a depletes carriers from a 2DEG underneath the Be:GaNmaterial 42′ at zero gate bias as denoted by the removal of the dashesin the depiction of the 2DEG 23 in FIG. 4B.

Next, and referring to FIG. 4C, the hard mask 52′ is lifted off alongwith the deposited polycrystalline Be:GaN material 42′p leaving theepitaxial Be:GaN material 42′a in the AlGaN recess region 56′, as shown.

Next, and referring to FIG. 4D, the epitaxial Be:GaN material 42′a inthe AlGaN recess region 56′ is patterned lithographically and etched,here for example using a dry plasma etching process, to form a structurehaving a lower base portion 42BASE and a thicker, vertically projecting,mesa shaped, portion 42M, over where the epitaxial Be:GaN gate region42′ is to be formed, as shown. The thickness of the Be:GaN material 42′ain the base portion 42BASE should be nominally as close to 0 Å aspossible without etching the AlGaN layer 24 below the base portion42BASE. Etching into the AlGaN layer 24 below the base portion 42BASEwill reduce the total current that can be passed in the source and drainaccess regions of the E-Mode HEMT 16′.

Referring now to FIG. 4 , the D-Mode HEMT 12 (FIG. 1 ) and E-mode HEMT16 are completed using any conventional processes; here however, thesource and drain electrodes of the E-mode HEMT 16′ are formed on theBe:GaN base portion 42BASE and in ohmic contact with the GaN channellayer 22 while the gate electrode 40′ is formed with a Schottky metalcontact 42 b in direct contact with the vertically projecting, mesashaped, epitaxial doped Be:GaN portion 42M.

Referring now to FIG. 5 a simplified diagrammatical sketch of asemiconductor structure 10″ having both the D-Mode HEMT 12 and an E-ModeHEMT 16″ according to an alternative embodiment is shown. Here,referring to FIG. 5A, after forming the structure shown, and describedabove in connection with FIG. 3C, a hard mask 62 is lithographypatterned to have a window 54″ formed therein region where the E-modeHEMT 16″ is to be formed.

Here, referring to FIG. 5B, an ion implantation process is used toimplant ions, here for example nitrogen ions, into a region 70 of theGroup III-N layers 20, 22, and 24 not covered by the hard mask 62 tomake the Group III-N material electrically resistive. The depth of theimplanted region 70 is determined by the type of ion used and theacceleration energy and should extend through the AlGaN layer 24, theGaN channel layer 22, and into the upper portion of the doped GaN bufferlayer 20. The implanted region 70 terminates below the depth of the 2DEGchannel 23 and minimizes mobile carrier conduction in the portion of the2DEG channel 23 below the E-mode HEMT 16″, being formed.

Next, referring to FIG. 5C, a layer of GaN 72 is grown by MBE over theupper surface of the structure shown in FIG. 5B, followed by an MBEgrown layer of AlGaN 74. It is noted that the portions of the GaN layer72 and the portions of the AlGaN layer 74 deposited on the hard maskform as polycrystalline layers 72 p while the portions of the GaN layer72 deposited on the AlGaN layer 24 grow epitaxially and the portions ofthe AlGaN layer 74 deposited on the GaN layer 72 grow epitaxiallyforming a 2DEG channel indicated by the dotted line 73 in the GaN layer72, as shown. It is noted MBE epitaxially regrown GaN layer 72 is indirect contact with the ion implanted region 70.

Next, referring to FIG. 5D, the hard 62 is etched away using for examplea buffered oxide wet etch to remove a dielectric masking layer, alongwith the polycrystalline layers 72 p on the hard mask 62, as shown.

Referring now to FIG. 5E, a hard mask 76 is formed over the surface ofthe structure. Again, the hard mask 76 is a non-reactive dielectric suchas, for example, SiN_(x), Al₂O₃, SiO₂ or a stable metal.

Referring now to FIG. 5F, the hard mask 76 is patterned with a windowover the portion of the AlGaN layer 74 where a gate electrode is to beformed, as shown, and using an etchant, here for example, a dry plasmaetch process and a recess 78 is etched into the upper portion of theAlGaN layer 74, as shown, so that only 50 Å to 180 Å of the AlGaN layer74 remains.

Referring now to FIG. 5G, a layer 80 of doped GaN, here Be:GaN, is grownby MBE over the upper surface of the structure shown in FIG. 5F; it isnoted that the portions of the doped GaN layer 80 deposited on the hardmask 76 form as polycrystalline layer 80 p while the portions of thedoped GaN layer 80 deposited on the AlGaN layer 74 grow epitaxially assingle crystal GaN, as indicated.

Next, referring to FIG. 5H, the hard mask 76 is etched away, using forexample a buffered oxide wet etch to remove a dielectric masking layer,along with the polycrystalline layer 80 p on the hard mask 76, as shown.Next, referring to FIG. 5 , the D-mode HEMT and E-Mode HEMT processingis performed in parallel to form the structure 10″ shown, having aD-Mode HEMT 12 with source, drain and gate electrodes 26, 28, 34,respectively, as shown, and an E-Mode HEMT 16″ having source, drain andgate electrodes 36′, 38′ and 40″, respectively, as shown; the gateelectrode 40″ being formed with a Schottky metal contact 82 in directcontact with the epitaxial doped Be:GaN material 80.

Referring now to FIG. 6 a simplified diagrammatical sketch of asemiconductor structure 10′″ according to an alternative embodiment isshown. Here, and referring to FIG. 6A, after forming the structureshown, and described above in connection with FIG. 5B, a GaN layer 72 isgrown by MBE in direct contact with the ion implanted region 70,followed by the MBE growth of an AlGaN layer 74, and then the MBE growthof a doped GaN layer 90, for example single crystal beryllium doped GaN(Be:GaN), as shown. The AlGaN layer 74 must be kept thin enough so thatthe doped GaN layer 90 can deplete the carriers in the 2DEG region 73′that would otherwise form at the interface of the GaN layer 72 and theAlGaN layer 74. It is noted that the portions of the GaN layer 72, AlGaNlayer 74 and the doped GaN layer 90 form as a polycrystalline layer 90 pon the mask 62, as indicated in FIG. 6A; however, the portions of theGaN layer 72, the AlGaN layer 74 and the doped GaN layer 90 deposited onthe AlGaN layer 24 form as single Group III-N layers.

Next, referring to FIG. 6B, the hard mask 62 is etched away using forexample a buffered oxide wet etch to remove a dielectric masking layer,along with the polycrystalline layer 90 p on the hard mask 62, as shown.

Next, referring to FIG. 6C, the doped GaN (Be:GaN) layer 90 is patternedlithographically and etched, here for example using a dry plasma etchingprocess, to form a structure having a lower base portion 90BASE and athicker, vertically projecting, mesa shaped, portion 90M, over where theepitaxial Be:GaN gate region is to be formed, as shown. The thickness ofthe dope GaN (Be:GaN) material 90 in the base portion 90BASE should benominally as close to OA as possible without etching the AlGaN layer 74below the base portion 90BASE. Etching into the AlGaN layer 74 below thebase portion 90BASE will reduce the total current that can be passed inthe source and drain access regions of the E-Mode HEMT 16′″.

Next, referring to FIG. 6 , the D-mode HEMT 12 and E-Mode HEMT 16′″processing is performed in parallel to form the structure 10′″ shown,having a D-Mode HEMT with source, drain and gate electrodes 26, 28, 34,respectively, as shown, and an E-Mode HEMT 16′″ having source, drain andgate electrodes 36, 38 and 40′″, respectively, as shown; the gateelectrode 40′″ being formed with a Schottky metal contact 82 in directcontact with the epitaxial doped GaN mesa portion 90M. It is noted thatthe GaN layer 72 serves as a channel layer and the AlGaN layer 74 servesas a barrier layer.

Referring now to FIG. 7 a simplified diagrammatical sketch of asemiconductor structure 10″″ according to an alternative embodiment isshown. The structure is fabricated in a similar manner as thealternative embodiments shown in FIG. 5 and FIG. 6 , where a fraction ofthe original 2DEG channel 23 is processed such that the 2DEG is nolonger electrically conductive and a new Group III-N structure is grownon the electrical inactive 2DEG channel 23 for the purposes of formingan E-mode HEMT 16″″. FIG. 7 shows that this processing allows fordifferent channel materials 22′, 72′ and barrier materials 24′, 74′ tobe used in a D-Mode HEMT 12′ and the E-mode HEMT 16″″. The ability touse different layer constructions in E-mode and D-mode HEMTs on the samewafer allows the materials to be tailored to provide a larger rangeelectrical performance.

A number of embodiments of the disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the disclosure. Forexample, while Be doped GaN has been described, it should be understoodthat other Group III-N doped material may be used such as doped AlGaN.Similarly, the use of different Group III-N channel and barrier layersthan GaN and Al_(0.25)Ga_(0.75)N will require E-mode gate regions withdifferent thicknesses for the barrier layer and the beryllium dopedGroup III-N layers to achieve a desired threshold voltage. Although SiCsubstrates have been used to illustrate various embodiments of thedisclosure, the disclosure does not depend on the use of any specificsubstrate and can be applied to any D-Mode Group III-N HEMT materialwhether it is grown on a substrate, for example, Si, Al₂O₃, and GroupIII-N, or if the HEMT is free-standing or mounted to another substrate.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A method comprising: forming an Enhancement-Modehigh electron mobility transistor (HEMT) structure having a AlGaN/GaNstructure to produce a 2DEG in a GaN portion of the AlGaN/GaN structure,the forming comprising: forming a gate structure for theEnhancement-Mode HEMT structure comprising: using a molecular beamepitaxy (MBE) process to form a beryllium-doped GaN layer formed undergallium-rich growth conditions to produce a resistive material thatshifts a band structure in the Enhancement-Mode HEMT structure toproduce positive threshold voltages required for E-mode operation,wherein the beryllium-doped GaN layer extends into a AlGaN portion ofthe AlGaN/GaN structure.
 2. The method recited in claim 1, wherein theberyllium-doped GaN layer is grown by the MBE process with apredetermined gallium to nitrogen flux ratio selected to maintain morethan a monolayer of liquid gallium on a surface of the AlGaN portionduring the MBE process.
 3. A method comprising: forming anEnhancement-Mode high electron mobility transistor (HEMT) comprising apair of stacked Group III-Nitride semiconductor layers, the pair ofstacked Group III-Nitride semiconductor layers forming a heterojunctionwith a 2DEG channel being formed in a lower one of the pair of stackedGroup III-Nitride layers, the forming comprising: forming a gateelectrode comprising a layer, disposed between an electricallyconductive gate electrode contact and a gate region of theEnhancement-Mode HEMT, the layer comprising a Group III-N materialhaving a predetermined resistivity; and depositing by molecular beamepitaxy, a dopant in the Group III-N material, wherein the dopant:provides the layer with a resistivity greater than the predeterminedresistivity of the Group III-N material; and depletes carriers from the2DEG channel at zero gate bias, and wherein the layer extends into anupper one of the pair of stacked Group-III nitride layers.
 4. The methodrecited in claim 3 wherein the dopant comprises beryllium.
 5. A methodcomprising: forming an Enhancement-Mode high electron mobilitytransistor (HEMT) comprising a pair of stacked Group III-Nitridesemiconductor layers, the pair of stacked Group III-Nitridesemiconductor layers forming a heterojunction with a 2DEG channel beingformed in a lower one of the pair of stacked Group III-Nitride layers,the forming comprising: forming a gate electrode comprising a layer,disposed between an electrically conductive gate electrode contact and agate region of the Enhancement-Mode HEMT, the layer comprising a GroupIII-N material having a predetermined resistivity; and depositing bymolecular beam epitaxy, a dopant in the Group III-N material, whereinthe dopant: provides the layer with a resistivity greater than thepredetermined resistivity of the Group III-N material, and depletescarriers from the 2DEG channel when an applied gate voltage is less thana threshold voltage, and the threshold voltage is equal to, or greaterthan zero, and wherein the layer extends into an upper one of the pairof stacked Group III-Nitride semiconductor layers.
 6. The method recitedin claim 5 wherein the dopant comprises beryllium.